1. Field of the Invention
The present invention relates to a webbing take-up device, and more specifically to a webbing take-up device provided with a lock mechanism that is designed to, when certain conditions such as a sudden vehicle deceleration are fulfilled, block rotation in the direction in which webbing is pulled out of a take-up shaft having a webbing belt wound in layers thereon for restraining a vehicle occupant.
2. Description of the Related Art
There are webbing take-up devices that are made to be attached to a vehicle. These are equipped with lock mechanisms that block rotation of a take-up shaft, on which a webbing belt is wound in layers, in the direction in which the webbing is pulled out when the vehicle is experiencing an emergency. An example of such a device is disclosed in Japanese Patent Publication No. 10-500648. The lock mechanism has, for example, a lock gear that rotates coaxially with a take-up shaft and which has ratchet teeth formed on the outer periphery thereof. Further, the lock mechanism also has a lock plate that corresponds to the lock gear and which can engage the ratchet teeth of the lock gear. There are also other devices in which the lock mechanism is not provided with a lock gear, rather, the mechanism is designed with a lock plate to be able to directly engage the take-up shaft.
A webbing take-up device provided with such lock mechanisms has an acceleration sensor that detects when the vehicle is in a state of sudden deceleration. The acceleration sensor detects the rate of acceleration (deceleration) of the vehicle and when the deceleration exceeds a certain amount, it operates the lock mechanism. When it has been detected with the acceleration sensor that the rate of deceleration of the vehicle has exceeded the certain amount, the lock mechanism operates and the lock plate latches the take-up shaft either directly or indirectly (via the lock gear). The lock plate stops the rotational force (i.e., load) of the take-up shaft in the direction in which the webbing is pulled out. Thus, the device is configured to block rotation of the take-up shaft in the direction in which the webbing is pulled out.
The lock plate of such a webbing take-up device is usually manufactured with a process that uses, for example, die casts. It is preferable that the lock plate of the webbing take-up device be made as strong as possible in order to be able to stop the rotational force of the take-up shaft turning in the webbing pullout direction.
Nonetheless, with die cast parts, the parts need to be made larger and heavier in order to obtain strength that is equal to that of parts that are press formed. For this reason, there has been a need to increase the strength of the lock plate (i.e., the strength that stops the rotational force of the take-up reel rotating in the webbing pullout direction) without increasing the size or weight of the lock plate.